1. Field of the Invention
The present invention relates to an angular position selecting apparatus in a variable damping-force type suspension system in which the cross-sectional area of oil path in a shock absorber is varied in accordance with the angular position of output shaft of an actuator.
2. Prior Art
FIG. 9 shows a conventional angular position selecting apparatus and FIGS. 10 and 11 illustrate the operation thereof. Referring to FIGS. 9-11, a permanent magnet 3 having two pairs of magnetic poles (i.e., four poles) is secured to an output shaft 6. A stator 7 is in coaxial relation with the magnet 3 so that salient poles 7a-7f are disposed circumferentially around the magnet 3 with some clearance between the magnet 3. The magnet 3 serves as a rotor, and is adapted to drive the shaft 6 into rotation when the stator 7 is energized.
The six salient poles 7a-7f are disposed in a diametrically opposed relation with each other, opposite poles being of the same magnetic polarities. The respective salient poles are each mounted a first to six windings 8-13 thereon, each of which forming an electromagnet. The six windings are connected to form a three-phase Y-connection as a whole as shown in FIG. 4. Between each and the next salient pole is provided with an opening (7g-7l). The winding 8 mounted on the pole 7a is in series with the winding 11 mounted on the pole 7d. Likewise, the winding 9 mounted on the pole 7b is in series with the winding 12 mounted on the pole 7e, and the winding 10 mounted on the pole 7c in series with the winding 13 mounted on the pole 7f.
A current-controlling circuit 14 is connected with a selector switch 18. The selector switch 18 incorporates three switching elements 18a-18c for the respective modes of operation of the shock absorber; hard (H), medium (M), and soft (S). When an operator closes the switching element corresponding to a desired mode of operation, a current is supplied to corresponding terminals to cause the actuator output shaft 6 to rotate through a predetermined angle, which in turn changes the cross-section of the oil path in the shock absorber. The current-controlling circuit 14 is supplied power by a switch 7 and a fuse 16 connected in series with the positive terminal of a battery E. The battery E has its negative terminal grounded.
The operation of the conventional angular position selecting apparatus in FIGS. 9-10 will now be described with respect to a case where the apparatus is switched from the soft mode to the hard mode. When the operator operates the selector switch 18, the switching element 18c becomes open and switching element 18a closed. This permits the current-controlling circuit 14 to supply a positive voltage to a terminal 15a and a negative voltage to a terminal 15b. Then, a current flows through the terminal 15a, coil 8, coil 11, coil 12, coil 9, and terminal 15b, causing the poles 7a and 7d to become an "N" and the poles 7b and 7e to become an "S."
Shortly after the selector switch 18 is operated to shift from the soft mode to the hard mode, the permanent magnet 3 or rotor is at a position shown in FIG. 10A where the pole 7a (N) repels the pole N1 of the rotor 3 while the pole 7d repels the pole N2 of the rotor 3. Further, the pole 7b (S) repels the pole S2 of the rotor 3 while the pole 7e attracts the pole N2 of the rotor 3. Moreover, the pole 7b (S) attracts the pole N1 of the rotor 3 while the pole 7e repels the pole S1 of the rotor 3. As a result, a clockwise torque acts on the rotor 3 to cause the output shaft 6 to rotate therewith.
Thus, the rotor 3 rotates through 60 degrees from the position in FIG. 10A and comes to rest at an angular position, as shown in FIG. 10B, where the pole S1 of the rotor 3 faces and attracts the pole 7a (N) and the pole S2 of the rotor 3 faces and attracts the pole 7d (N). At this angular position of the rotor 3, the pole N1 of the rotor 3 faces and attracts the pole 7b (S), and the pole N2 of the rotor 3 faces and attracts the pole 7e (N).
Should the rotor 3 overrotate clockwise due to inertia, the pole S1 of the rotor 3 moves close to the pole 7b to repel each other while at the same time the pole S2 of the rotor 3 moves close to the pole 7e (S) to repel each other. This exerts a counterclockwise torque on the rotor 3 so that the rotor 3 returns where it is supposed to be.
Conversely, should the rotor 3 overrotate counterclockwise for some reason, the pole N1 of the rotor 3 repels the pole 7a (N) and the pole N2 of the rotor 3 repels the pole 7d (N). Also, the pole S1 of the rotor 3 is pulled back by the pole 7a (N), and the pole S2 of the rotor 3 is pulled back by the pole 7d (N).
Consequently, the torque acting on the rotor 3 always tends to hold the rotor 3 at the angular position such that the opening 7g is aligned with the rotor 3 at the part where the poles N1 and S1 are put together, and the opening 7j is aligned with the rotor 3 at the part where the poles N2 and S2 are put together.
The other modes can be selected by applying voltages of proper polarities to the terminals 15a-15c as shown in Table 1 via the current-controlling circuit 14.
TABLE 1 ______________________________________ Mode terminal 15a terminal 15b terminal 15c ______________________________________ soft (-) none (+) medium none (+) (-) hard (+) (-) none ______________________________________
Friction forces and the force of inertia in the oil path impose heavy loads on the actuator when the rotor 3 starts to rotate. As depicted by curve B shown in FIG. 8, the above-described conventional apparatus exhibits a maximum torque at an angle of about 15 degrees from the position where the rotor 3 is at rest in any mode, and shows a very small torque when the rotor 3 starts to rotate. A small torque may take longer time for the rotor 3 to settle at a desired angular position or may be too small for the rotor 3 to rotate.